The present invention relates to an improved solid state system for providing direct current to a single or plurality of load elements from a three phase power supply system according to a predetermined current level sequence, and also includes the provision of a number of protective alarm circuits for insuring proper current flow through the load elements.
There are a great number of applications for applying direct current through a load or a plurality of load elements according to a predetermined current level sequence. For example, in the nuclear art it is necessary to raise and lower control rods within the reactor core for controlling the amount of energy output from the nuclear reactor. The use of the term "control rods" is used here to include any member positioned within the reactor which alters the reactivity of the reactor. Thus, this includes rods which serve other purposes besides normal control use. The use of the word "rod" is synonymous with "control rod" for the purpose of this invention. The control rods are located in proximity with nuclear fuel elements consisting of nuclear fissionable fuel. Generally, the greater the number of neutrons in the reactive region, the greater the number of fissions of the fuel atoms take place, and consequently the greater the amount of energy is released. Energy, in the form of heat, is removed from the reactive region by a coolant which flows through the region and then flows to a heat exchanger wherein the heat from the reactor coolant is used for generating steam for driving turbines to transform heat energy into electrical energy. To decrease the energy output of the nuclear reactor, the control rods, made of material which absorbs neutrons, are inserted within the reactive region. The greater the number of control rods and the farther the control rods are inserted within the reactive region, the greater number of neutrons will be absorbed and hence the energy output of the reactor will be decreased. Conversely, to increase the energy output of the nuclear reactor, the nuclear control rods are withdrawn from the reactive region; consequently the number of neutrons absorbed decreases, the number of fissions increases, and the energy output of the reactor increases. Control rods typically are arranged in banks, with each bank comprising a number of groups of control rods. Because of safety considerations, extremely reliable control rod systems must be used.
One system presently used to lower and raise the control rods has incorporated a jack-type electromechanical mechanism which employs a plurality of electrical coils to incrementally insert or withdraw each control rod within the reactor. These incremental steps are repeated usually by groups within a bank or banks, as many times as necessary in order to move the control rods to a position which produces the desired output from the reactor.
One such jack mechanism is more fully described in U.S. Pat. No. 3,158,766 issued to E. Frisch and assigned to the assignee of the present invention. The jack mechanism disclosed in the Frisch patent includes three inductive coils, one for gripping, one for lifting and one for holding the control rod in a stationary position. Thus when there are a given plurality of control rods within the reactor, there will be a corresponding number of gripper, lift, and stationary holding coils operative with those control rods. It is desirable, and in fact mandatory, that the rods be lifted in a predetermined order so that no one rod is above or below any of the others in its group. The current requirements for each of the coils within a group will be the same at all times for all of the corresponding coils i.e., in all of the lift, gripper and the stationary coils.
One method for supplying current to each of these types of coil or load elements employs electromechanical means. Such a system is shown, for example in U.S. Pat. No. 3,099,778, issued to W. R. Kennedy et al and assigned to the assignee of the present invention. In that patent, electromechanical contacts are sequentially opened and closed by a motor driven cam contact arrangement to provide full voltage to the coils for the entire period that they were energized.
There are many disadvantages of this type of electromechanical current for sequencing such a plurality of load elements. First, no current regulation is provided by such a scheme. The contactors merely close and open the circuit thereby directing all or none of the current from the power source through each of the groups of load elements or coils, and therefore no current regulation is provided. The current conducted by the coil depends on the inductance and resistance of the coil. Cold coils conduct considerably more current. In the case of a nuclear reactor, current levels have been increased substantially in control rod systems, particularly in control rod jack mechanisms in order to achieve faster rod movement. In a nuclear control rod jack mechanism the steady state current for one jact lift coil is approximately 90 ampls at 125 volts under cold conditions and 51 amps at 125 volts under hot conditions. Secondly, the energy stored in the inductive coils must be dissipated when the contactor is opened, and thus most energy is wasted. Because of the high energy stored in the coils, damage to the electromechanical contacts due to arcing and insulation breakdown due to voltage transients is common and the possiblity of equipment failure exists. The higher power dissipation in the coils as a result of the current level increase has made current regulation highly desirable or even necessary. Lowering the dissipation in the coils increases insulation life and decreases supply requirements. Third, it is often desired to energize and deenergize each group of inductive load elements very rapidly and the resulting operating rate and power dissipation can be beyond the capability of electromechanical switching components presently used. Finally, in the past art devices, no provision has been made to check to see that the current called for through each of the load elements, or, as in the case of the nuclear control rod system, the current through each group of jack mechanism coils, has in fact gone through these loads, or has exceeded the time period during which such currents are required. As a result, it has been possible to burn out the load elements necessitating expensive and often burdensome repairs, particularly in a nuclear reactor where such repairs result in an expensive shutdown of the whole nuclear plant.